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What color are the plugs now? What color were the old ones when removed?
While engine is running, pull each plug wire one at a time and reinstall to see if engine will stay running.
Sounds like you have a coil not firing as much . One side hotter than the other. ( giving plugs diff colors once removed) You are could also be getting more fuel to one side than the other and causing it to burn rich causing the black plug. I would regap the coil on the side that is black to make sure it has proper coil gap and fire. If it continues to burn rich, I would replace that coil.
Let me back up..... This a single or dual coil engine? What engine is on this machine?
what is the spec number of engine? Does it have 1 or 2 coils?
Okay, no problem.
You have a good evening also.
you have 2 coils on this machine. I would re-gap them both just to be safe
remove the top engine cover to display the coils. use a business card to place between the coils and the magnets of the flywheel. I can send a video or go on you tube and search how to gap mower coil and there will be many videos as visuals help me more at times